Q: How is it possible that a hybrid auto achieves better fuel mileage in city driving than highway driving tests? Regenerative braking does not convert 100% of the car's KE to recharge the battery. Are the batteries fully charged at the conclusion of the city test by the gasoline engine? Is that fuel accounted for? Isn't it true that ALL the energy used to propel the car is from the gasoline engine (either to power the car or to generate electricity to charge the batteries so that they can power the car)? It seems to defy the laws of physics.

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A: It's simpler than that. Remember aerodynamic drag (the major fuel consumer at highway speeds) goes up with the cube of velocity. The highway mileage cycle is at higher average speed where fuel economy is poorer. Non-hybrids get poor fuel economy in urban cycles than on the highway because they are wasting energy slowing down and dissipating the energy in the brakes. Hybrids recover some of that energy, and the overall mileage at low speeds is improved. If the average speeds on the two cycles were the same, the urban cycle would still show poorer economy.

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Q: When old tires are burned they emit thick black smoke. But when Nascar cars slide sidewise or do victory burn-outs the smoke is pure white. Why the difference?

A: When tires smoke from spinning on the pavement, they're not actually on fire. The temperatures are too low. The white smoke is vaporized rubber. The black smoke from burning tires is the carbon that's left over from incomplete combustion.

Q: i have a 1994 Chevy 1500/4x4/350 pickup truck with 130,000 miles on it. At times it runs somewhat hot at over 200 degrees. If I turn on the heater the temp. goes down. Is this just a bad thermostat or is the fan clutch failing? I did a listen test for the fan clutch. There is no noise coming from the fan area. It seemed to rev up when the temp was near 200, and the temp. dropped to 170 and stayed there. With a cold engine I did the spin test on the fan and it spun 4 to 5 times with slight resistance. Any ideas?

A: This sounds like the fan clutch to me. Does it run hot on the freeway? Moist vehicles will run fine at 60 or more with no fan at all. So if it's okay at speed and overheats at idle, it's more likely the clutch. A fan clutch shouldn't take more than an hour to replace.

Q: I've been wondering how long lithium batteries will last in electric cars—the batteries themselves, not their charge. I discovered a support document from electronics company Fujitsu that stated batteries only last 2-3 years after manufacture and are good for 300-500 charges including partial charges. So with this formula if you charge your car every night either partially or fully, the battery will last only last 2-3 years. If batteries to make a really small car go 75 miles cost about $10,000, there's no way electric cars can be viable. I think electric cars use exactly the same batteries as laptops—I know the Tesla does. What do you think?

A: The lithium batteries used in automotive applications are based on a different chemistry than the lithium-ions used in cellphones, laptops and power tools. (Except for Tesla's battery). They have better long-term stability, and the charge-discharge rates and how deeply they are discharged or recharged are controlled carefully by the on-board electronics. So, they should last a lot longer that 300-500 cycles. Or so I'm told. We'll see what experience in the field says. The longevity and cost of the Ni-Mh batteries used in the Prius and other hybrids has exceeded my predictions of a couple of years ago.

Q: i have a 1997 Mitsubishi Eclipse Spyder. i love this car, but I have an annoying problem that Mitsubishi engineering doesn't make easier. The problem is that the antenna won't retract. I hear the motor working after the engine is started and again when you turn it off. I'm assuming there is something broken that's probably plastic. In the trunk there is a hand hole in the plastic to change lamps for the tail lights, but this opening doesn't extend forward far enough to get to the power antenna. What do i need to do?

A: Try this first. Clean the mast with a rag and some mineral spirits or lacquer thinner, and then coat it lightly with Vaseline or mineral oil or even silicone spray. Then help the mast down while someone else pushes the switch. Cycle the mast up and down a half-dozen times to distribute the lubricant. If that doesn't work, you will almost certainly have to replace the antenna motor/mast assembly.